Hierarchical modulation multiplexing (using SCM) is a technique for performing multiplexing transmission by hierarchizing modulated signals of different users. For example, Non-Patent Document 1 discloses this hierarchical modulation multiplexing transmission. This document discloses multiplexing and transmitting a modulated signal of high transmission power for a distant user (i.e. MS #1) and a modulated signal of low transmission power for a nearby user (i.e. MS #2) from a base station apparatus (“BS”) (see FIG. 1). Note that, upon SCM, a user of a high transmission power coefficient is a “distant user,” and a user of a low transmission power coefficient is a “nearby user.”
As described above, with hierarchical modulation multiplexing transmission, there is a ratio of transmission power between a modulated signal for a distant user and a modulated signal for a nearby user (that is, the difference in transmission power of both), and therefore it is assumed that modulated signals for different users can be separated on the receiving side.
FIG. 2 shows a signal point constellation for a combined signal combining two modulated signals. Here, assume that transmission power coefficients for the distant user (i.e. MS #1) are α, and transmission power coefficients for the nearby user (i.e. MS #2) are β. Further, the ratio of transmission power of both modulated signals is α2:β2=4:1. The modulated signal for the distant user (i.e. MS #1) is modulated by QPSK, and the modulated signal for the nearby user (i.e. MS #2) is modulated by 16 QAM.
A combined signal is acquired by applying weights to modulated symbols (where the modulated symbol for the distant user is S1 and the modulated symbol for the nearby user is S2) addressed to both users according to the ratio of transmission power and adding the modulated symbols. That is, modulated signal S subjected to hierarchical modulation multiplexing is represented by S=αS1+βS2.
The receiving side equalizes a channel fluctuation on the channel for the received signal, and then detects a signal point (i.e. symbol detection) based on the signal point constellation shown in FIG. 2.
As shown in FIG. 2, if α is greater than β, signal points in the signal point constellation for the combined signal do not overlap each other. Further, when the modulated symbol (i.e. S2) for the nearby user is focused upon, the minimum inter-signal point distance is the same in the case where S2 alone is transmitted and in the case where S2 is combined with S1 and transmitted.
By contrast with this, when a modulated symbol (i.e. S1) for the distant user is focused upon, the minimum inter-signal point distance is smaller in the case where S1 is combined with S2 and transmitted than in the case where S1 alone is transmitted. As a result, there is a possibility that receiving performance deteriorates. However, by making α substantially greater than β (α>>β), it is possible to minimize the influence of deterioration of receiving performance.
In view of above, conventional hierarchical modulation multiplexing transmission (using SCM) allows the receiving side to detect only the desired symbol even if not only the desired symbol but also signals addressed to other users are included. That is, it is possible to assign extra transmission power to other users upon downlink transmission from a base station apparatus to terminal apparatuses, so that this contributes to increasing the number of users that can be accommodated and improving throughput of a wireless communication system.    Non-Patent Document 1: IEEE 802.16 Broadband Wireless Access Working Group, written contribution, IEEE C802.16m-07/116